The present invention relates to the generation of new wavelengths by nonlinear effect, in particular for continuous-wave lasers of low or medium power (typically <5 W).
Nonlinear processes are very practical means of generating wavelengths that are not easily accessible from more common wavelengths. Second order generating processes such as second harmonic generation or sum and difference frequency generation are known. Third order generating processes are also known such as, for example, stimulated Raman scattering, inter-alia.
For example, it is quite easy to produce lasers at 1064 nm, the materials and optical pumps necessary for this being known to those skilled in the art and available thereto. By second harmonic generation, it is possible to obtain an emission at 532 nm, then an emission at 266 nm via a second stage of second harmonic generation or an emission at 355 nm by summing the frequencies of the fundamental wave (1064 nm) and the doubled wave (532 nm).
For any laser assembly, it is accepted that its efficiency, representing the output power as a function of the pump power, must be optimized.
However, the efficiency of nonlinear processes depends on the intensity of the optical signals. It is very high (often higher than 50%) with laser emissions originating from Q-switched lasers the peak powers of which generally exceed a kW. Many commercial devices have been available for a long time.
This efficiency is in general low with conventional nonlinear crystals and optical powers of about a few watts or less.
By way of example, using one of the most efficient materials for second harmonic generation at 1064 nm, KTP, the conversion efficiency hardly reaches 0.02% with a power of 1 W at 1064 nm focused into a 50 μm beam at the center of a 5 mm-long KTP crystal. A 200 ρW emission at 532 nm results. This efficiency is even lower for second harmonic generation of an emission at 532 nm in BBO, to produce an emission in the deep UV (266 nm).
To increase the efficiency, it is possible to envision decreasing the size of the beam by further focusing. However, this decreases the useful nonlinear crystal length, because of the strong divergence of the beam.
Recent work has allowed the efficiency of certain nonlinear crystals to be improved. Quasi-phase matching developed on lithium niobate (ppLN) or KTP (ppKTP) allows conversion efficiencies of about 1% to be obtained for 1 W of signal at 1064 nm. However, these materials are not for example available in the UV. ppSLT exists for frequency summation between 1064 nm and 532 nm, but its efficiency is not very high.
The most effective way of enhancing the nonlinear effect is to include the nonlinear crystal in a resonant cavity resonant at the wavelength of the fundamental wave.
This is the case when the nonlinear crystal is included in the laser cavity. Solid-state lasers emitting at 532 nm or at 561 nm are an example of the inclusion of a nonlinear crystal in the laser cavity. For example, Nd:YAG pumped at 808 nm is an amplifier at 1064 nm or 1123 nm.
Intracavity doubled argon (gas) lasers, for example emitting at 244 nm, are another example thereof.
When it is not possible to insert the nonlinear crystal into the laser cavity, and if the laser is single-frequency, it is possible to inject the laser emission into an external cavity and to adjust the optical length of the external cavity so as to make it resonant with the laser emission. At resonance, the power of the fundamental wave is typically amplified by a factor S (ratio of the intra- and extra-cavity power, called la facteur de surtension in French). The phase width of the resonance is about 2π/S. In the prior art, this has conventionally been how a continuous-wave source at 266 nm has been obtained.
Finesses of about 30 to 100 are usually employed in nonlinear external cavities in order to achieve conversion efficiencies of from 10% to more than 30%.
External cavities are usually produced by assembling and aligning at least two mirrors (Fabry-Perot cavity) and often four mirrors (bow-tie ring cavities). In such prior-art systems, any mechanical movement due for example to a mechanical vibration induces a modification of the phase of the optical wave. A movement δ of one of the mirrors of a Fabry-Perot cavity induces a phase variation of 4πδ/λ, which must be much smaller than 2π/S if it is desired to avoid fluctuations in the power of the fundamental wave in the cavity (and therefore of the wave generated by the nonlinear effect). Therefore, for the external cavity to operate correctly, δ<<λ/2S. For a wavelength of 500 nm and a finesse of 50, the mechanical fluctuations must therefore respect δ<<5 nm! For a cavity of 50 mm length, this amounts to a relative stability of better than 10−7. This mechanical stability is unachievable with conventional mechanical systems.
FIG. 1 shows a prior-art system for generating laser beams by nonlinear effect. A bow-tie ring cavity comprising four mirrors and an LBO crystal may be seen. The length of the cavity is servocontrolled to the power of the pump, i.e. the incident laser beam. To do this, some of the incident beam is sampled by reflection from a first mirror. The sampled beam is then detected by one or more photodetectors via a quarter-wave plate and optionally a beam splitter. The photodetectors supply a processing unit that generates a control signal for controlling the movement of a second mirror by means of a piezoelectric module so as to modify the length of the cavity.
In the prior art, the cavities used for second harmonic generation are therefore electromechanically servocontrolled, in general piezoelectrically and using an error signal generating method such as that of Hansch-Couillaud (such as illustrated in FIG. 1) or that of Pond-Drever-Hall.
However, external cavities have technological limitations.
The piezoelectric actuators used require voltages of about a kV. The associated electronics become complex and expensive as soon as the passband frequencies are higher than of the order of a kHz. The maximum voltage of the piezoelectric actuator corresponds to the maximum intensity of mechanical vibration that it is possible to correct. The maximum frequency corresponds to the maximum frequency of vibration that it is possible to correct. Therebeyond, the servocontrol system falls out of sync.
It is known from the publication W. Kozlovsky et al., “Efficient Second Harmonic Generation of a Diode Laser Pumped CW Nd:YAG laser using monolithic MgO:LiNbO3 external resonant cavities”, IEEE JQE vol 24, p 913, that when the fundamental source is insensitive to vibrations (for example with a monolithic laser) and the external cavity is also monolithic, it is possible to “thermally” servocontrol the external cavity to the frequency of the fundamental source. This solution is elegant and easy to implement, assuming it is possible to produce the external cavity and the source.
However, monolithic external cavities themselves also have limitations.
Firstly, external cavities are closed by two reflective mirrors and require at least one focal element. In a monolithic external cavity, the mirrors are deposited directly on the nonlinear crystal and the focusing element is obtained with a polish with a curvature. Many nonlinear crystals and in particular those that are effective in the UV such as BBO have thermal expansion coefficients that are too high for it to be possible to deposit dielectric mirrors thereon. Furthermore, they are often difficult to polish (hygroscopicity, etc.) and this makes it difficult to achieve a polish with a radius of curvature. This polish is thus expensive and of poor quality. It is therefore often impossible or not economical to produce an external monolithic cavity.
Secondly, many nonlinear crystals have a high sensitivity to temperature. This therefore requires a preciser control of temperature.
Thirdly, when the temperature acceptance of the nonlinear crystal is low, it may not be possible to find a temperature that makes the cavity resonant and that is located in the phase matching range of the nonlinear crystal.
Fourthly, many nonlinear crystals exhibit substantial walk-off, limiting the nonlinear conversion efficiency and degrading the quality of the converted beam.
Finally, since servocontrol of temperature is naturally slow, it is difficult to avoid oscillations in the power output from the external cavity.
The objective of the present invention is to mitigate the aforementioned drawbacks by providing an external cavity that is highly effective. Another aim of the invention is to make it possible to generate wavelengths that are unusual for a low-power continuous-wave laser, in particular wavelengths in the UV.